Fix a bad linked list insertion algorithm.
Fix a bad linked list insertion algorithm.

The insertion into the thread_infos linked list was only correct for
the first and second insertions.  After that, it would always chop list
list off at two entries.  This caused all sorts of strange behavior
and crashes when more than two threads (signal handlers, process
waiters, etc.) were in use.

The fix just inserts the new thread_info onto the head of the list.

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